It is known in the door control art to provide means for generating a brief audible warning substantially simultaneous with the closing of a subway door, elevator door or the like. See, for example, U.S. Pat. No. 4,546,845 to Meyer et al which discloses the use of an audible warning signal which sounds for a brief interval immediately prior to the closing of a train car door.
Train car doors, subway doors and elevator doors generally slide into an enclosure in the opening direction and, hence, represent little or no danger to adjacent persons. For this reason, the audible signal is operated only immediately prior to or simultaneous with activation of the doors in the closing direction.
An entirely different set of circumstances prevails in the case of a gate of the type which translates by rolling or sliding parallel to and immediately adjacent a section of stationary fence to open and close a break or opening in the fence. In this case, the "scissors action" of the exposed sliding or rolling gate relative to the immediately adjacent stationary fence section represents a continuous danger to adjacent personnel while the gate is translating in both the opening and closing directions. Moreover, the risk of injury along the adjacent section of stationary fence, a distance which may in some cases be on the order of 20 or 30 feet, is immediate as soon as the gate begins to translate and persists throughout the translation.